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Car Batteries

Community Veteran
Posts: 2,930
Thanks: 171
Fixes: 3
Registered: 05-04-2007

Car Batteries

Just wanted a bit of advice from someone
who knows more about car mechanics than me (i.e. more than zero).

So have a 10 year old Merc, battery died (not surprised given the age) in Janurary, get new one from car spares place literally 5 mins from me. Had to get a friend to help me fit it, which we did and was a pain to say the least (trying not to swear here).

Since we know the square root of nothing between us regarding cars, I was impressed we did it. Got there in the end, but it was a mission. Don't need to go to a Gym just try to carry that car battery 5
mins down the road, it was a monster.
I haven't used the car for around 5 weeks, battery dead. Had to use the manual key thingy to get into the car and boot (fun). Found my RAC charger.

Plugged it in, it's got some options such as 6v Trickle Charge, 12v Trickle Charge and 12v Fast Charge. I assume fast is bad news as it just nukes the battery ASAP in case you're in trouble away from home.

Anyway luckily I wasn't so chose the 12v Trickle Charge.

The Amp meter didn't even increase when I connected it. So I thought "Have I done it right", power light was on and the RAC charger said green light meant it was charged (which it isn't).

Yes at least I managed to get the positive to negative bit right, but it has gone from near enough nothing to 2A which I assume is a good thing? It is taking charge and the battery hasn't died. Leave it for as long as possible?
7 REPLIES
Community Veteran
Posts: 2,286
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Registered: 04-08-2009

Re: Car Batteries

First, have you checked that the battery fluid levels are ok? Two amps is next to nothing for a dead battery if it's healthy. You should be able to prize off the green Lucas thingies on the top to expose the cells, and you should be able to see that the fluid actually covers the plates inside each cell. If not, disconnect the charger, get some distilled water and top it up. 

What tends to happen with old batteries is that the plates get covered in lead sulphate which insulates the plates and will render them knackered. They will appear white as opposed to a dark colour. If that's happened it's fresh battery time I'm afraid. 

If not, and the fluid is up to scratch, try putting it on boost charge. It shouldn't do any harm because the car's own alternator charges at a pretty fast rate. Trickle charge means just that, a trickle to keep the battery topped up, because it will lose charge gradually in any case by itself. I would expect the initial current to be up to 8 amps, dropping back to about 4 for a good battery that's absorbing energy.

Community Veteran
Posts: 2,930
Thanks: 171
Fixes: 3
Registered: 05-04-2007

Re: Car Batteries

Thanks Nozzer,

Trouble is it a six month old new battery, which worked and now doesn't. I don't want to tamper with it too much as it has a 3 year warranty, so it leaves them room to question me (they seem okay but just in case).

EDIT: Will test the nuke charge, thanks.
maranello
Aspiring Pro
Posts: 932
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Registered: 11-01-2008

Re: Car Batteries

There are lots of reasons for a battery going flat, and a few you can check yourself.

1. Alternator belt - is this loose? if there is more than an inch or so of defection possible on the longest stretch between pulleys, then the belt needs tightening.

2. Alternator - if you have a voltmeter check the voltage across the battery terminals when the engine is running. It should be around 14 volts.

3. Battery - is is a normal lead-acid type or a more modern AGM. it should say on the battery. These need a specialist battery charger.

4. Battery charger - trickle charge should ideally be used for ~ 24 hours on a lead-acid battery.

If you're not confident with checking/testing these yourself, you could try asking the retailer you bought the battery from (most branches of Halfords can do a battery/alternator check). Obviously you need to get enough charge in the battery to get it started. 

My other car isn't a Ferrari
Minivanman
Hero
Posts: 4,794
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Registered: 04-11-2014

Re: Car Batteries

Ditto to all that, especially the voltage check across the terminals.

All views expressed are my own but you can express them too if you want to be right about everything like I am.
Community Veteran
Posts: 2,286
Thanks: 219
Fixes: 1
Registered: 04-08-2009

Re: Car Batteries

@Alex

If the battery is new, chances are that something is drawing current in the car while its stationary. Central locking? Door lock not making the stop contact? Draws current just sitting there. When (if) you get the battery charged, try feeling the door locks (including the boot) to see if any are warm. This has happened to me on three occasions. 

RobPN
Aspiring Pro
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Registered: 17-05-2013

Re: Car Batteries

@Alex  As you've probably managed to get some charge into the battery, it might be worth checking the voltage across the terminals with the charger disconnected (leave it for a few minutes before checking).  If the voltage is much lower than 12 volts, say 10.x volts or 8.x volts, that would indicate one or more failed cells.  Probably unlikely on a newish battery, but possible.

Edit: Typo

 

 

Community Veteran
Posts: 2,930
Thanks: 171
Fixes: 3
Registered: 05-04-2007

Re: Car Batteries

Thanks all,

I put the charger into nuke mode for about 2 hours, then the car started as if nothing happened at all. When you try to open the doors via the remote after a flat battery the lights blink a lot, which I am sure is sarcastic to say "I'm back alive".

Good point though, I will double check there is nothing killing it. Obvious ones are headlights and interor lights I guess.
Main pain was finding a spanner to get the bolts off.

I'll give it local run tomorrow to give it a bit of charge and I'm not stranded too far from home.